1. Field of the Invention
The present invention relates to breathing circuits affording a re-breathing capability and, more specifically, to reliability and safety enhancements to apparatus employed to divert an exhaled breath volume for re-breathing by a patient and to subsequently remove such volume from the breathing circuit after re-breathing.
2. State of the Art
A so-called xe2x80x9cairwayxe2x80x9d valve having a re-breathing mode and installed in a ventilator or other breathing circuit (the term xe2x80x9cventilatorxe2x80x9d being used generically herein to encompass various types of breathing circuits) selectively controls the diversion of an exhaled breath volume from the primary passage of the circuit into a xe2x80x9cdeadspacexe2x80x9d volume defined by a chamber or other vessel such as a loop of hose for subsequent re-breathing by the patient. The re-breathing of the CO2-laden exhaled breath volume initiates a change in respiratory CO2 concentration which may be employed to estimate cardiac output in a non-invasive manner. A discussion of a partial re-breathing technique wherein an additional, fixed deadspace is intermittently and briefly introduced into the ventilator circuit is discussed in detail in Capek, J. and Roy, R., xe2x80x9cNoninvasive Measurement of Cardiac Output Using Partial CO2 Rebreathing,xe2x80x9d IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 35, NO. 9, SEPTEMBER 1988, pp. 653-661, the disclosure of which is hereby incorporated in its entirety by this reference.
An airway valve employed to divert an exhaled air volume into the deadspace volume, to subsequently add the diverted volume into the ventilator circuit for re-breathing and then to remove it from the breathing circuit requires a high degree of reliability. Specifically, failure to remove the added volume after an appropriately brief period of time results in an increased volume of inspired CO2, with an attendant higher level of ventilation and arterial CO2.
An exemplary breathing circuit including a deadspace volume for partial re-breathing defined by a loop of hose is schematically illustrated in FIG. 1 of the drawings. Exemplary breathing circuit 500 includes a tubular airway 502 that communicates air flow to and from the lungs of a patient. Tubular airway 502 may be placed in communication with the trachea of the patient by known intubation processes, by connection to a breathing mask positioned over the nose and/or mouth of the patient, by a mouthpiece for the patient or via an endotracheal tube. A flow meter 504, which is typically referred to as a pneumotachometer, and a carbon dioxide sensor 506, which is typically referred to as a capnometer, are disposed between tubular airway 502 and a length of hose 508, and are exposed to any air that flows through breathing circuit 500. Suitable pneumotachometers are disclosed in U.S. Pat. Nos. 5,379,650 and 5,535,633, and a suitable capnometer is disclosed in U.S. Pat. No. 5,793,044. If desired, a combined air flow and carbon dioxide sensor such as that disclosed in U.S. Pat. No. 5,789,660 may be employed in lieu of discrete flow and gas sensors. Both ends of another length or loop of tubing 510, which may be referenced as defining a deadspace or re-breathing volume 512, communicate with hose 508. Deadspace volume 512 may optionally include an expandable section 514, which may be provided by the use of corrugated tubing for tubing loop 510. A Y-piece 516, disposed on hose 508 opposite flow meter 504 and carbon dioxide sensor 506, facilitates the connection of an inspiratory hose 518 and an expiratory hose 520 to breathing circuit 500 and the flow communication of the inspiratory hose 518 and expiratory hose 520 with hose 508.
The two ends of tubing loop 510 defining deadspace volume 512 are connected to a two-mode airway valve 550, the two modes being a normal operating mode and a re-breathing mode. During normal breathing, airway valve 550 is maintained in the normal operating mode to prevent inhaled and exhaled air from flowing through deadspace volume 512. Airway valve 550 may be selectively actuated to shift from the normal operating mode to the re-breathing mode to divert a volume of a patient""s exhaled breath into deadspace volume 512, the breath volume being subsequently removed from deadspace volume 512 for re-breathing by the patient. Subsequent to re-breathing, airway valve 550 is shifted back to the normal operating mode so that the re-breathed air volume is expired through hose 508 and expiratory hose 520. During inhalation, gas flows into inspiratory hose 518 from the atmosphere or a ventilator (not shown). Processing unit 522 (preferably included within a patient monitor and hereinafter referred to as a xe2x80x9cmonitor processing unitxe2x80x9d) processes air flow and carbon dioxide input signals from flow meter 504 and 506 (or preliminary processing units associated therewith as known in the art), and preferably directly or indirectly controls operation of airway valve 550 to shift same between the normal operating mode and the re-breathing mode.
Airway valves such as valve 550 illustrated in FIG. 1 may be controlled pneumatically via a control line (tubing) which actuates the valve employing an actuation energy source comprising either a positive air pressure (i.e., a pressure greater than the internal breathing circuit pressure) or a negative air pressure (i.e., a partial vacuum lower than internal breathing circuit pressure). Thus, there is always a risk of a leak, tubing disconnect, pump failure, power loss or, however unlikely, a valve component jam or failure. Accordingly, it would be desirable to provide enhanced assurance that the expired breath volume added to the circuit from the deadspace volume is removed from the circuit by appropriate switching of the airway valve, by reversion of the airway valve to a normal operating mode upon partial or total failure of the actuation energy source or delivery system, and by alerting the clinician to any problems with the actuation or control of the airway valve.
It would also be desirable to afford enhanced reliability to a variety of apparatus which may be employed to provide a deadspace volume or otherwise cause re-breathing of a patient""s CO2-laden exhalations.
The present invention includes methods and apparatus for enhancing reliability of, and monitoring, the operation of various apparatus for providing a breathing circuit with a re-breathing capability. As used herein, the term xe2x80x9cbreathing circuitxe2x80x9d includes and encompasses any apparatus through which a patient or other subject may breath, such as, without limitation, ventilator breathing circuits, masks, mouthpieces, and endotracheal tubes.
In one aspect of the invention, fluid control line pressure (positive or negative) for actuation of a pneumatic airway valve for diverting an exhalation into a tubing loop or other receptacle or element defining or providing a deadspace volume may be specified as a selected pressure or within a selected range and monitored.
In a positive pressure pneumatic system, pressure reduced below a selected threshold may be compensated by actuation of a pump or a vessel containing compressed air, while pressure elevated above a selected threshold may be compensated by a bleed valve open to the ambient environment.
In a negative pressure pneumatic system, pressure elevated above a selected threshold may be compensated by actuation of a vacuum pump or opening of a valve connected to a vacuum line, while pressure reduced below a selected threshold may be compensated by opening an inlet valve to the ambient environment.
Monitoring of control line pressure may be effected on an intermittent (periodic sampling) or continuous basis and a controller, or processor such as a patient monitor processor linked to the controller, programmed so as to warn the user of any deviation from a selected pressure, a selected pressure range, or pressure deviations of selected magnitudes or frequencies or a combination thereof.
It is also contemplated that hydraulic, electrical, magnetic, mechanical and light or other radiation sources may be employed as driving energy sources to actuate an airway valve or other apparatus for providing a deadspace and monitored in appropriate ways to provide enhanced reliability according to the invention.
Furthermore, hardware and software xe2x80x9cwatchdogsxe2x80x9d may be incorporated into the controller for the valve or the monitor processing unit with which such controller is associated in order to preclude a software error from inadvertently causing an airway valve or other apparatus initiating re-breathing to maintain the breathing circuit in the re-breathing state.
In another aspect of the invention, the volume or level of CO2 inspired by the patient may be monitored using a sensor which measures both air flow and CO2, or individual air flow and CO2 sensors. Detection of excessive inspired CO2 volume triggers a warning. Similarly, end-tidal or end-inspired CO2 concentration or other appropriate measures of CO2 measured with a CO2 sensor may be employed as a warning trigger.
In still another aspect of the invention, monitoring of the correct operation of an airway valve may be effected by using the patient monitor to analyze CO2 or other gas waveforms (such as, for example, O2 or N2), either alone or optionally in combination with air pressure or flow waveforms (or both) already being processed for other purposes to ascertain whether the response expected for a particular airway valve mode (normal operating or re-breathing) is actually being produced. If the response is not as expected, an alarm or other alert may be generated to alert the operator. Thus, the system is xe2x80x9cself-checkingxe2x80x9d, in that the monitor is able to ascertain whether the airway valve actually did shift from one mode to another, responsive to the applied actuation pressure, and if a leak, blockage or mechanical failure has occurred in the airway valve actuation system or perhaps the valve itself. In this embodiment, no actual monitoring of control line pressure would be required, although it is contemplated that such monitoring in combination with waveform analysis might be effected for redundancy.